1. Field of the Invention
The present invention relates to a multiplex transmission apparatus with multiplex transmission lines capable of transmitting a variety of types of signals.
2. Description of the Related Art
Recently vehicles have realized an increasing number of on-board electronic devices. These devices are mounted in such vehicles due to advances in electronic control systems. Consequently, the number of wire harnesses used in vehicles has increased significantly. To decrease the number of wire harnesses used, a multiplex transmission system which transmits a variety of types of signals through a single wire harness has been widely adopted.
In the past, the present applicant has proposed a multiplex transmission apparatus including at least two wire harnesses (Japanese Patent Application Laid-open No. 4-95545). In the multiplex transmission apparatus described in the JP-'545 Publication one of two wire harnesses is used as a high-speed multiplex communication line to exchange signals of more importance in controlling the running conditions of a vehicle, such as signals for ABS systems (anti-lock brake system), EGI system (fuel injection control), TRC system (traction control), 4WS (4-wheel steering), and ACS system (active suspension). The other wire harness, on the other hand, is used as a low-speed multiplex communication line to exchange signals that need not operate instantaneously, such as signals for lamps, an air-conditioner, wipers, an audio system, power windows, and signals for meters, such as a water temperature meter and a speedometer.
The high-speed and low-speed multiplex communication lines are connected via gateway nodes, and the signals are exchanged between the two communication lines through these gateway nodes. In such case, a plurality of gateway nodes are connected between the two lines so that even if one gateway node malfunctions, the signals can be exchanged between the two communication lines through another gateway node.
In the above conventional multiplex transmission apparatus, however, since the high-speed and low-speed multiplex communication lines are connected via a plurality of gateway nodes, identical signals may be repeatedly transmitted from one communication line to the other. This brings about a problem of a low transmission efficiency of signals when, for example, the quantity of signals to be transmitted is large.
In addition, if identical signals are repeatedly transmitted in a multiplex transmission apparatus in which the transmission of signals through gateway nodes is performed by using ON/OFF switches, a switch to be turned on may be turned off or vice versa, impairing the reliability of the apparatus.